Introduction of air into injection water

ABSTRACT

Air is introduced into the injection water for microbial enhanced oil recovery. The injection water is passed through an ejector where it is entrained and the oxygen subsequently dissolves in the water.

The present invention relates to the introduction of air into water,particularly injection water used in oil recovery.

When oil is present in subterranean rock formations such as sandstone orchalk, it can generally be exploited by drilling into the oil-bearingmeasures and allowing existing overpressures to force the oil up theborehole. This is known as primary removal. When the overpressureapproaches depletion, it is customary to create an overpressure, forexample by injecting water into the formations to flush out standingoil. This is known as secondary removal.

However, even after secondary removal, a great deal of oil remains inthe formations; in the case of North Sea oil, this may represent 65% to75% of the original oil present. Of this remaining oil probably morethan half will be in the form of droplets and channels adhering to therock formations that have been water-flooded and the remainder will bein pockets which are cut off from the outlets from the field.

Several enhanced oil recovery methods have been proposed to exploit theaccessible but adhering oil remaining in the rock formations, one ofwhich is microbial enhanced oil recovery (MEOR). This entails the use ofmicro-organisms such as bacteria to dislodge the oil, and a number ofsystems have been proposed. In the case of consolidated measures, onesuch system employs aerobic bacteria.

The absence of any oxygen in oil bearing formations means that if anaerobic system is to be used, then oxygen must be supplied. However,when aerobic bacteria are used and oxygen (or air, containing oxygen) isinjected into the formation, the situation may not be satisfactory.Firstly, there is an immediate separation into a gaseous and an aqueousphase, which makes control of the system very difficult and in practice,limits the system to batch-type operation. Secondly, a great deal ofheat is generated, which, in view of the oxygen-rich gaseous phase andthe readily available combustible material, presents a considerable riskof explosion. A cooling medium must therefore also be employed.

The solution to this problem is addressed in British Patent No. 2252342.In this case, the injection water used contains a source of oxygencapable of yielding at least 5 mg/l free oxygen.

Essentially, the system is operated as follows. A population of aerobicbacteria is introduced into the formation at a position spaced from aproduction borehole. The micro-organisms are adapted to use oil as acarbon source. Pressurised injection water is introduced into theformation via an injection borehole, the water including a source ofoxygen and mineral nutrients. The bacteria multiply using the oil astheir main carbon source and the oxygen in the injection water as theirmain oxygen source. In so doing, they dissociate the oil from the rockformation and the dissociated oil is removed via the production boreholeby the injection water.

The rate of growth of micro-organisms is of course dependent on theavailable oxygen. In general maximum growth is desired and therefore itis desirable to maintain a high oxygen concentration in the injectionwater (and clearly also in advancing biomass layer). In some situationshowever, for instance where it may be desirable to stimulate theproduction of surfactants, the level of oxygen in the water phase mightneed to be reduced in order to stress the micro-organisms into producingsurfactants.

A situation would normally be established in which the biomass layerforms a front between the oxygen-rich injection water andoxygen-depleted water on the outlet side of the front. Initially, theoxygen-depleted water will be the formation water or oxygen freeinjection water but as the process progresses, it will be displaced byinjection water, stripped of its oxygen as it passes through the biomasslayer. Where the biomass is in contact with oil and has access tooxygen, it will feed on the oil, thereby dissociating the oil from therock by one or more of a number of mechanisms. The principal mechanismis believed to be the production of surfactants which reduce the forcesattaching the oil to the rock. The pressure of the injection water thenforces the oil out of the rock pores and the oil is carried forwards bythe injection water.

Normally, sea water for example would be expected to carry about 6 mg/lof oxygen in solution. In order to provide the bacteria with itsrequired oxygen source, a significant amount of oxygen must therefore beintroduced into the injection water. One way of achieving this would bewith the use of an air compressor. However, where the back pressures(well head pressures) are high, for example, above 8 atm (810 KPa), thecompressor required would be very costly. Furthermore, compressorsrequire servicing and are prone to failure, particularly when operatingat high pressures in demanding conditions.

It is therefore an object of the present invention to provide a systemfor introducing oxygen into water, particularly injection water for oilrecovery, in an inexpensive and reliable fashion.

It is a further object to enable the introduction to be achieved over avery large range of water back pressures.

According to the invention, there is provided the use of an ejector forintroducing oxygen into injection water for oil recovery in which theinjection water is supplied to the ejector at a predetermined pressureand oxygen, optionally as air, is also supplied to the ejector, thepressure and velocity of the water passing through the ejector beingarranged to draw oxygen into the water stream. The amount of oxygendrawn into the water is preferably capable of being dissolved entirelyat the wellhead (or formation) pressure as well as being sufficient toachieve the desired effect in the formation.

The ejector uses the energy of the injector pump to accelerate theinjection water, thereby reducing the pressure in order to draw in theair and requires a minimum of maintenance. It is very inexpensivecompared to a compressor, particularly in high wellhead pressureapplications. In addition, the use of an ejector enables very stableoxygen/water ratios to be achieved.

In marine situations, the injection water would be sea water.Preferably, the injection water is supplied at the predeterminedpressure by means of an injection pump. Preferably, the ejector islocated in the injection water line between the injection pump and thewell head. Alternatively, the ejector can be located at the watersuction side of the pump, particularly when the amount of oxygen to beintroduced is small, for example, less than 50 mg oxygen per litre ofwater.

The pump pressure may vary enormously in dependence upon the well headpressure. Thus, the pump pressure may range from 2 to 700 bar (0.2 to 70MPa). The injection pressure may vary from 0.9 to 350 bar (0.09 to 35MPa). The air:water ratio can also be varied considerably, dependingupon various factors, including the requirement of the micro-organismand the wellhead pressure, and a range of from 0.03:1 to 6:1 expressedin litres of air at normal conditions to litres of water.

The invention also extends to a method for introducing oxygen intoinjection water for oil recovery which comprises: supplying water to anejector by means of an injection pump; supplying oxygen, optionally asair, to the ejector; drawing oxygen into the water in the ejector. Theoxygen may then dissolve in the water downstream of the position wherethe air is introduced.

The invention also extends to apparatus for carrying out this method,which comprises an injector pump, a source of water, a source of oxygenand an ejector, and in which the source of water is connected to theinjector pump which supplies the water to the ejector and the source ofoxygen is also connected to the ejector; whereby the water passingthrough the ejector draws oxygen into the water.

Preferably, the injector pump is a high pressure pump. Preferably, theapparatus includes a water line bypassing the ejector, the bypass lineincluding a bypass valve. Preferably, the source of oxygen is an airline, the air line including a control valve and optionally a checkvalve. Preferably, the ejector is fitted with a check valve that closesat internal pressures greater than a given value, for example 0.9 bar(0.09 MPa). Preferably, the ejector is equipped with a passive or activeair flow control and measuring system.

Naturally, the ejector will be designed for the specific operatingconditions of each well/field, with regard to water volume, airconcentration and injection pressure.

Since the pressures involved with the injection water may be very high,the amount of gaseous oxygen that can be dissolved may be quiteconsiderable. The pressures encountered in some high pressureoil-bearing formations may be from 200 to 800 bar (20-80 MPa); at thesepressures up to 4.0 g of oxygen may be dissolved in a litre of water.This quantity is amply sufficient to allow aerobic bacteria to multiplyat a satisfactory rate with a bulk flow rate of the injection waterwhich is low enough to avoid reservoir damage.

Preferably, therefore, the amount of oxygen dissolved will be from 1mg/l to 4000 mg/l more preferably from 10 mg/l to 400 mg/l though theactual amount will be dependent upon the prevailing conditions. Theamount of oxygen present should not be as much as would be toxic to thebacteria.

In practice, the avoidance of a gas phase is very important sincemicrobial activity can only proceed in the liquid phase. Clearly, if agas phase is present, the oil adhering to the rock formation within thegas phase will remain unaffected by the micro-organisms.

The micro-organisms may be any convenient single-cell organisms such asyeasts but are most preferably bacteria. Suitable bacteria may bePseudomonas putida, Pseudomonas aeruginosa, Corynebacterium lepus,Mycobacterium rhodochrous, Mycobacterium vaccae, Acinetobacter andNocardia. The bacteria used may be pre-selected and cultivated to thrivein the injection water under the prevailing conditions.

The invention may be carried into practice in various ways and someembodiments will now be described by way of example with reference tothe accompanying drawings, in which:

FIG. 1 is a schematic diagram showing a water injection system for anoil well incorporating the introduction of air in accordance with theinvention; and

FIG. 2 is a schematic (section?) through a suitable ejector.

FIG. 1 shows an injection water line 11 directed to a wellhead (notshown). The water is supplied by means of an injection pump 12. Anejector 13 is located between the pump 12 and the wellhead. A bypassline 14 including a valve 15 bypasses the ejector and pressure gauges16,17 are located on the water line 11 on either side of the ejectorrespectively downstream of the bypass line inlet and upstream of thebypass line return.

An air line 21 is connected to the ejector 13. The air line 21 includesa flow meter 22, a control valve 23, a check valve 24 and a pressuregauge 25.

The ejector 13 is in the form of a jet pump. It comprises a first fluidinlet 31 for the air leading to a nozzle 32, and a second fluid inlet 33for the water. The air and water mix in the vicinity of the nozzle 32.Downstream of the nozzle 32, the ejector includes a venturi 34 leadingto an outlet 35.

In operation, the pump 12 operates at a constant speed, pumping water tothe wellhead, via the ejector 13. Air is drawn into the water stream atthe ejector 13 and dissolves in the water, by virtue of the high waterpressure, between the ejector 13 and the wellhead. The amount of airsupplied is adjusted using the control valve 23 and this is controlledin dependence upon the pressure in the air line 21 measured by thepressure gauge 25 and the pressure drop across the ejector 13 measuredby the pressure gauges 16,17. The amount of air drawn into the water isalso affected by the proportion of water which passes via the bypassline 14, thus avoiding the ejector 13.

En an alternative embodiment, for example, when the amount of oxygen tobe introduced into the water is small, typically less than 50 mg/l, theinjector 13 may be located on the suction side of the pump 12, togetherwith its bypass line 14 and valve 15.

The invention will be further illustrated in the following Example.

In one typical on-shore injection well, with a high wellhead pressure ofabout 68 bar (6.8 MPa), an injection pump is used which operates at 188bar (18.8 MPa). The pump supplies water at a rate of 40 l/min. Toachieve an air:water ratio of 1:1, an ejector 13 with a throat diameterof 2 mm is used, resulting in a water linear velocity of about 118 m/s.

What is claimed is:
 1. The use of an ejector for introducing oxygen intoinjection water for oil recovery, in which: injection water is suppliedto said ejector at a predetermined pressure and passes through saidejector as a stream; oxygen is also supplied to said ejector at anadjustably controlled rate; and the pressure and velocity of said waterstream passing through said ejector are arranged to draw said oxygeninto said water stream, wherein said controlled rate of oxygen supply iscontrolled based on feedback as to the pressure within the oxygen supplyline and the pressure differential across the ejector so as to result ina predetermined stable concentration of dissolved oxygen in saidinjection water.
 2. A use according to claim 1, in which said water issea water.
 3. A use according to claim 2, in which said injection wateris supplied at said predetermined pressure by means of an injectionpump.
 4. A use according to claim 3, in which said ejector is located inan injection water line between said injection pump and a well head. 5.A use according to claim 4, in which said ejector is on the suction sideof the injection pump.
 6. A use according to claim 3, in which thepressure of said injection pump is about 2 to about 700 bar (0.2 to 70Mpa).
 7. A use according to claim 3, in which the injection pressure isabout 0.9 to about 350 bar (0.09 to 35 Mpa).
 8. A use according to claim1, in which said oxygen is supplied as air.
 9. A use according to claim8, in which the air:water ratio after injection is about 0.03:1 to about6:1 expressed in liters of air at normal conditions to liters of water.10. A method for introducing oxygen into injection water for oilrecovery which comprises: supplying water to an ejector by means of aninjection pump; supplying oxygen to the ejector at an adjustablycontrolled rate; and drawing said oxygen into the water in the ejector,wherein said controlled rate of oxygen supply is controlled based onfeedback as the pressure within the oxygen supply line and the pressuredifferential across the ejector so as to result in a predeterminedconcentration of dissolved oxygen in said injection water.
 11. A methodaccording to claim 10, in which said oxygen is supplied as air.
 12. Amethod according to claim 10, in which said water is sea water.
 13. Amethod according to claim 10, in which the pressure of said injectorpump is about 2 to about 700 bar (0.2 to 70 MPa).
 14. A method accordingto claim 10, in which the injection pressure is about 0.9 to about 350bar (0.09 to 35 MPa).
 15. A method according to claim 10, in which theair:water ratio after injection is about 0.03:1 to about 6:1 expressedin liters of air at normal conditions to liters of water.
 16. Apparatusfor carrying out a method according to claim 10, which comprises: aninjector pump, a source of water, means for supplying oxygen at anadjustably controlled rate, and an ejector, and in which said source ofwater is connected to said injector pump, said injector pump suppliessaid water to said ejector, and said means for supplying oxygen is alsoconnected to said ejector; whereby said water passing through saidejector draws oxygen into said water, and whereby said means forsupplying oxygen may be adjusted so that said water has a predeterminedconcentration of dissolved oxygen.
 17. Apparatus according to claim 16,in which said injector pump is a high pressure pump.
 18. Apparatusaccording to claim 16, further comprising a bypass water line bypassingsaid ejector, said bypass water line including a bypass valve. 19.Apparatus according to claim 16, in which said means for supplyingoxygen is an air line, said air line including a control valve. 20.Apparatus according to claim 19, in which said air line further includesa check valve.
 21. Apparatus according to claim 16, in which saidejector is fitted with a check valve that closes at internal pressuresgreater than about 0.9 bar (0.09 MPa).